Incomplete Dominance, Codominance and ABO Blood Groups

A heterozygote need not resemble one parent

Mendel's tall/dwarf pea model shows complete dominance, but the same segregation of alleles can lead to different appearances. The genotype ratio from heterozygote selfing remains 1:2:1 because it results from gamete combinations. The phenotype ratio depends on what each genotype produces. Under incomplete dominance, the heterozygote lies between the two homozygotes for the selected trait; under codominance, both allele products are recognisable. Neither pattern involves alleles blending irreversibly inside gametes. Their reappearance in descendants demonstrates that hereditary information remains discrete even when expression differs from a simple dominant/recessive pattern.

Incomplete dominance in snapdragon

Cross a true-breeding red snapdragon RR with a true-breeding white snapdragon rr. The F1 genotype is Rr, but the flowers are pink rather than red. The heterozygote has an intermediate phenotype because the amount or effect of pigment produced by one functional allele is not equivalent to that produced by two. On selfing Rr, alleles segregate normally, giving 1 RR red : 2 Rr pink : 1 rr white. Here the genotypic and phenotypic ratios both equal 1:2:1. A pink flower is not produced by a permanently blended red-and-white allele; R and r remain distinct in its gametes.

What molecular mechanisms can underlie dominance?

Many gene products are enzymes, receptors or structural proteins, and their expression matters through biological pathways. If one functioning allele supplies enough product for a normal phenotype, a defective allele can appear recessive. If half the usual product creates a visibly different outcome, an intermediate heterozygote may result. Some alleles produce different functioning products that can both be detected; this is a basis for codominance. There is no one molecular mechanism explaining every dominant relationship. The observable trait and its measurement matter: a genotype may behave as dominant for visible seed shape yet show intermediate starch-grain size under microscopy.

ABO blood groups and three alleles

A single ABO locus has three widely taught allelic categories, Iᴬ, Iᴮ and i. Because each individual ordinarily carries two copies at this autosomal locus, a person can have one of six unordered genotypes: IᴬIᴬ, Iᴬi, IᴮIᴮ, Iᴮi, IᴬIᴮ or ii. The Iᴬ allele specifies an enzyme that modifies an RBC surface carbohydrate into A antigen; Iᴮ produces a different modification leading to B antigen; i usually lacks the corresponding functional activity. Iᴬ and Iᴮ are codominant to one another and each is dominant to i in the standard blood-group phenotype. Four blood groups arise from the six genotypes: A, B, AB and O.

Codominance versus multiple alleles

In an IᴬIᴮ individual, both A and B antigens can be detected on erythrocytes, rather than an intermediate 'half-A-half-B' new antigen. That simultaneous expression defines codominance. 'Multiple alleles' refers to having more than two common forms of a gene in the population, not having three allele copies within one ordinary diploid individual. The ABO system illustrates both terms without making them synonyms. A person with Iᴬi is group A; one with Iᴮi is group B; with ii is group O. These phenotype labels are useful for inheritance but do not by themselves determine compatibility in every transfusion situation, which also involves other blood-group antigens.

Worked family example

Suppose a group A parent is Iᴬi and a group B parent is Iᴮi. The first produces Iᴬ and i gametes with equal probability; the second produces Iᴮ and i gametes. The four offspring genotypes, each with probability 1/4, are IᴬIᴮ (AB), Iᴬi (A), Iᴮi (B) and ii (O). The possibility of a group O offspring reveals that both parents could transmit i. It would be impossible in the simple ABO model if the A parent were IᴬIᴬ, because no i allele could come from that parent. This is reasoning from inherited alleles, not from blood-group name alone.

One gene and more than one phenotype

A single gene may affect several measurable features because a protein participates in multiple processes. For pea seed starch synthesis, a functional allele can yield round seeds while the heterozygote shows intermediate starch granule size compared with the two homozygotes. The dominance category therefore depends on which biological outcome is studied. Another phenomenon, pleiotropy, occurs when one gene influences multiple distinct traits; we will study it alongside polygenic inheritance.

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